A fuel cell is an electric power generating system for directly converting chemical reaction energy of oxygen and hydrogen contained in a hydrocarbon-based material such as methanol, ethanol, or natural gas, into electrical energy.
Such a fuel cell is a clean energy source which can replace fossil energy, which has an advantage of providing a variety of range of power based on stack configuration via lamination of unit cells, and attracts much attention as a small portable power supply owing to 4 to 10-times higher energy density than small lithium batteries.
Representative examples of fuel cells include polymer electrolyte membrane fuel cells (PEMFCs) and direct oxidation fuel cells. A direct methanol fuel cell (DMFC) refers to a type of direct oxidation fuel cell which uses methanol as a fuel.
The polymer electrolyte membrane fuel cell has advantages of high energy density and high power, but has disadvantages of requiring care in handling hydrogen gas and additional facilities such as fuel modification devices for modifying methane, methanol, natural gas or the like in order to produce hydrogen as a fuel gas.
On the other hand, the direct oxidation fuel cell has lower energy density than the polymer electrolyte membrane fuel cell, but has advantages of easy handling of fuels, operation availability at a low operation temperature such as room temperature and in particular, and no need for fuel modification devices.
In such a fuel cell system, the stack actually generating electricity has a structure in which several to several dozen unit cells, each consisting of a membrane-electrode assembly (MEA) and a separator (also called a “bipolar plate”), are laminated. The membrane-electrode assembly has a structure in which a polymer electrolyte membrane including a hydrogen ion conducting polymer is interposed between an anode (also called a “fuel electrode” or “oxidation electrode”) and a cathode (also called an “air electrode” or “reduction electrode”).
Electricity is generated by a fuel cell based on the following principle. A fuel is supplied to a fuel electrode, i.e., the anode, is adsorbed on a catalyst of the anode, and is then oxidized to produce a hydrogen ion and an electron. The generated electron moves to an oxidation electrode, i.e., a cathode via an exterior circuit, while the hydrogen ion passes through the polymer electrolyte membrane and then moves to the cathode. An oxidizing agent is supplied to the cathode, the oxidizing agent, the hydrogen ion and electron react with one another on the catalyst of the cathode to produce water and, at the same time, generate electricity.
Research to improve activity of catalysts is actively underway because the performance of the fuel cell is greatly affected by the performance of catalysts of the anode and cathode.
In particular, the polymer electrolyte membrane fuel cell is commercially available and practically applicable earlier than other fuel cells due to the advantage of providing high-efficiency high power at low operation temperature.
Cost reduction via reduction of the amount of platinum used is the most potential issue in order to commercialize polymer electrolyte membrane fuel cells. However, reduction in amount of platinum used can have adverse influences on both power and durability.
Generally, a Pt/C catalyst wherein nano-scale Pt is supported on carbon having a high specific surface area is most commonly used, but there is a problem of decrease in durability caused by deterioration of the catalyst.
Therefore, research is continuing to design structures of catalyst layers capable of solving problems associated with performance and durability resulting from decreased platinum content.